For clarification of common terms used in this document, an overview of certain cellular telecommunication system configurations is presented in the following.
Proposals for third-generation systems include UMTS (Universal Mobile Telecommunications System) and FPLMTS/IMT-2000 (Future Public Land Mobile Telecommunications System/International Mobile Telecommunications at 2000 MHz). In these plans cells are categorised according to their size and characteristics into pico-, nano-, micro- and macrocells, and an example of the service level is the bit rate. The bit rate is the highest in picocells and the lowest in macrocells. The cells may overlap partially or completely and there may be different terminals so that not all terminals necessarily are able to utilise all the service levels offered by the cells.
FIG. 1 shows a version of a future cellular radio system which is not entirely new compared with the known GSM system but which includes both known elements and completely new elements. In current cellular radio systems the bottleneck that prevents more advanced services from being offered to the terminals comprises the radio access network RAN which includes the base stations and base station controllers. The core network of a cellular radio system comprises mobile services switching centres (MSC), other network elements (in GSM, e.g. SGSN and GGSN, i.e. Serving GPRS Support Node and Gateway GPRS Support node, where GPRS stands for General Packet Radio Service) and the related transmission systems. According e.g. to the GSM+ specifications developed from GSM the core network can also provide new services.
In FIG. 1, the core network of a cellular radio system 20 comprises a core network CN 931 which has three parallel radio access networks linked to it. Of those, networks 932 and 933 are UMTS radio access networks and network 934 is a GSM radio access network. The upper UMTS radio access network 932 is e.g. a commercial radio access network, owned by a telecommunications operator offering mobile services, which equally serves all subscribers of said telecommunications operator. The lower UMTS radio access network 933 is e.g. private and owned e.g. by a company in whose premises said radio access network operates. Typically the cells of the private radio access network 933 are nano- and/or picocells in which only terminals of the employees of said company can operate. All three radio access networks may have cells of different sizes offering different types of services. Additionally, cells of all three radio access networks 932, 933 and 934 may overlap either entirely or in part. The bit rate used at a given moment of time depends, among other things, on the radio path conditions, characteristics of the services used, regional overall capacity of the cellular system and the capacity needs of other users. The new types of radio access networks mentioned above are called UMTS terrestrial radio access networks (UTRAN). Such a network can co-operate with different types of fixed core networks CN and especially with the GPRS network of the GSM system. The UMTS terrestrial radio access network (UTRAN) can be defined as a set of base stations (BS) and radio network controllers (RNC) that are capable of communicating with each other using signalling messages.
The terminal 10 shown in FIG. 1 is preferably a so-called dual-mode terminal that can serve either as a second-generation GSM terminal or as a third-generation UMTS terminal according to what kind of services are available at each particular location and what the user's communication needs are. It may also be a multimode terminal that can function as terminal of several different communications systems according to need and the services available. Radio access networks and services available to the user are specified in a subscriber identity module 936 (SIM) connected to the terminal.
FIG. 1 further shows some details of the structure of a radio access network. A radio access network 932, 934 typically comprises one or more base stations 937 and a controlling unit 42. In UMTS radio access networks 932, 933 the controlling unit is called the radio network controller (RNC), and in GSM networks 934 the controlling unit is called a base station controller (BSC). The radio access networks typically comprise also other network elements such as transcoder units. FIG. 1 further shows a mobile services switching centres (MSC) 43 which basically controls circuit-switched connections of mobile stations 10 and a Serving GPRS Support Node (SGSN) 41 which basically controls packet switched connections of mobile stations 10.
In cellular telecommunication systems, connections are set up and resources reserved on demand and released when not needed. Moving mobile stations place additional requirements for the system, connections need to be transferred from one base station to another, which often requires a substantial change in the routing of the connection through the network. The exchanges of information between various network elements which are necessary for executing such functionality are handled by mobility management (MM) protocols and radio resource control (RRC) protocols. Mobility management protocols mainly take care of the mechanisms allowing a mobile station to move within the cellular network and security issues, while RRC protocols mainly take care of controlling the use of radio resources over the air interface, the connection between a mobile station and the BSC/RNC, and handovers. The execution of the RRC protocols is mainly performed by the mobile station and the BSC/RNC. Some parts of the RRC protocol, such as functionality related to inter-MSC handovers, are executed by the MSC. The execution of the MM protocols is mainly performed by the mobile station and the MSC. The protocols used in the GSM system are described further in the book “The GSM System for Mobile Communications” by Michel Mouly and Marie-Bernadette Pautet, ISBN 2-9507190-0-7, Palaiseau 1992.
In principle, the MM and RRC protocols are mainly separate. However, in order to reduce signalling, many MM functions are started or finished as a result of events in the RRC level, without explicit messaging in the MM level. For example, during releasing of a connection in the present GSM system, the MM protocol changes its state from a WAIT FOR NETWORK COMMAND state to the IDLE state as a response to releasing of the radio resources in the RRC level. Therefore, the releasing of radio resources in the RRC level implicitly acts as a control signal for the MM level. However, this approach creates problems in the new cellular systems under development, such as the UMTS system. Some of these problems are described in the following with reference to FIG. 2.
FIG. 2 shows a mobile station 10 and the cellular network 20, various entities taking care of executing the protocols such as the MM protocol entity 11 and the RRC protocol entity 12 of the mobile station. At the network side, corresponding entities are the MM protocol entity 21 in the MSC and the RRC protocol entity 22 in the RNC. The MM and RRC entities are typically realized using software programs executed by a processing unit such as a microprocessor in the control unit of a MS or a network element.
Some services take care of the mobility management of their connections themselves, independently from the MSC. An example of such services is the GPRS service. Their independent mobility management results in the execution of a further MM protocol, which is separate from the MM protocol executed by the MSC. In GPRS, the SGSN executes its own MM protocol. The second MM protocol has corresponding MM protocol entities 13, 23 in the MS and in the SGSN. However, only one RRC protocol is used, since it is advantageous to have the control of all radio resources needed by the mobile station in a single protocol, and in a single controlling entity at each end of the radio connection. For example, it is advantageous to use only one ciphering mode for all transmissions of the mobile station, instead of negotiating and using one ciphering mode for connections via the MSC, and another ciphering mode for packet connections via the SGSN.
Since only one RRC protocol is used, MM level mechanisms based on implicit meanings of events in the RRC level do not work properly, when two MM protocols are used. For example, if packet transmission bearers are released while speech bearers are still active, at least some RRC connections 30 still remain in use. Since the packet connection MM entity 13 of the mobile station expects to observe the releasing of RRC connections as an indication of releasing of the MM connection 32, 34, it remains in the WAIT FOR NETWORK COMMAND state, since the RRC connections are still in use by the connections controlled by the other MM protocol 11, 31, 33, 21.
Problems also arise during establishment of connections. During the setup of a connection, the MM protocol authenticates the terminal and controls the starting of ciphered mode communication over the air interface. For example, if a packet data connection is set up when speech connections are already in use, the communication over the air interface is already ciphered, wherefore no ciphering is started for the new packet connection. However, since the reception of a ciphering mode command by the MS is interpreted as an indication of a succesfully performed set up of a MM level connection, the MM entity 13 in the MS 10 does not receive any indication of the setting up of the MM level connection.